The present invention relates to compositions and methods for accelerating decoke operation of transfer line exchangers (TLE) in steam crackers for olefin production. Particularly, the compositions and methods disclosed relate to introducing decoke enhancers by atomized injection into TLE inlet cone before and/or during furnace decoke operation. More particularly, the decoke enhancers are aqueous solutions of metal chromates and dichromates, or metal manganates and permanganates, or metal carbonates, or metal acetates and oxalates, or metal hydroxides, or their mixtures thereof. Additionally, the said compositions and methods are applicable to both shell-and-tube and double-pipe TLE""s which are commonly used in steam crackers for olefin production.
In a typical steam cracking furnace, a cracked hydrocarbon stream leaves furnace coils at a temperature ranging from 750 to 850xc2x0 C. and enters immediately the TLE""s, where the hot process stream is cooled rapidly from typically 750xc2x0 C. to about 300xc2x0 C. There are two types of TLE""s which are very commonly used in industrial steam crackers for ethylene production: shell-and-tube TLE""s and double-pipe TLE""s. A shell-and-tube TLE has three main sections: the entrance cone, the tubesheet and tubes, and the exit cone, while a double-pipe TLE has mainly one section of a-pipe-in-a-pipe configuration.
Coke deposition in steam cracking furnaces is an inevitable process, reflecting the chemistry and nature of cracking reactions of hydrocarbons. Although coke deposition occurs in furnace coils, especially in the high temperature radiant section, it also happens in TLE""s operated at lower temperatures. Particularly, coke deposition can become a very severe problem in a shell-and-tube TLE due to its geometric configuration. Additionally, with heavy feedstocks such as naphtha, the low operating temperatures (650-300xc2x0 C.) in a TLE can induce substantial condensation of high boiling components from the cracked hydrocarbon stream. Then, the formed condensates in TLE can undergo a dehydrogenation process and form solid coke deposits.
Due to the inevitable coke build-up in the radiant coil and TLE""s, steam cracking furnaces can normally operate for typically 20-60 days and a decoke operation has to take place to remove the coke deposits. A typical decoke operation involves passing air and steam through the furnace coils and TLE""s which are maintained at more or less the same temperature range as during cracking operation. After 2-3 days, the coke deposits in the furnace coils can be removed (combusted or gasified) almost completely. However, for TLE""s, such decoke operation often can not remove the coke deposits completely since the TLE operating temperatures are too low for combustion/gasification reactions to proceed to completion. Therefore, coke deposits accumulate fairly rapidly in TLE""s and after a few cycles of coking-decoking operation (typically 3-4 months), the TLE""s together with the whole furnace must be brought offline, cooled and the TLE""s must be cleaned mechanically. This operation not only requires high maintenance costs but also cause interruptions to production for typically about 4-10 days. The present invention discloses a method to accelerate decoke operation for TLE""s as well as the compositions of the decoke enhancers. Therefore, the overall TLE run length before a mechanical decoke can be prolonged and very likely mechanical decoke for the TLE can be eliminated. In addition, the injected decoke enhancer can also reduce coke formation in the TLE during the subsequent cracking operations and therefore extend the overall runlength of a steam cracker.
To date, different inhibitors to reduce coke formation in the furnace coils have been patented [U.S. Pat. No. 6,228,253 of Zalman Gandman, U.S. Pat. Nos. 4,889,146 and 4,680,421 of David Forester, U.S. Pat. Nos. 5,330,970 and 4,724,064 of Dwight Reid]. Reports on accelerators to gasification of coke in furnace coils can also be found in literatures [Dave Kesner et al, Chemical Technology Europe, Sep/Oct. 94, pp14-16, and S. E. Babash et al, PTQ Autumn 99, pp113-120]. However, there is hardly any prior art available on decoke enhancers for TLE""s.
U.S. Pat. No. 6,228,253 issued May 8, 2001 to Zalman Gandman discloses an injection nozzle for injecting additives into the coils of a pyrolysis furnace. The body of the specification discloses injecting salts of group IA (group 1) and group IIA (group 2) in a polar solvent into the coils. The patent discloses the salts may be tetrasilicates, tetraborates, pentaborates, borates, nitrates, potassium liquid glass and boric acid. The patent fails to teach the use of chromate salts or carbonates as required in the present invention. Further the patent does not disclose or suggest injecting such mixtures into transfer line exchangers.
U.S. Pat. No. 4,889,146 issued Dec. 26,1989 to Betz Laboratories, Inc. discloses treating pyrolytic reactors and furnaces with alkali metals, preferably magnesium, acetates, chlorides and nitrates and magnesium sulfate. The reference fails to teach the use of group 1 or 2 chromates and dichromates nor does the reference relate to treating transfer line exchangers.
U.S. Pat. No. 5,330,970 issued Jul. 19, 1994 to Betz Laboratories teaches that a mixture of a boron compound and a dihydroxybenzene compound may be added to the steam or feedstock to a heated metal surface to reduce or inhibit coke formation. The boron compound may be ammonium borate, biborate, pentaborate, boron oxide or sodium borate. The dihydroxybenzene compound may be hydroquinone, resorcinol, catechol, or 4-tert-butyl resorcinol. The mixture may be added to the steam or the feedstock. The reference fails to teach the use of group 1 or 2 metal chromates and dichromates nor does the reference teach the application of these types of systems to transfer line exchangers.
There are a number of patents which teach the use of boron compounds to inhibit coke formation on heated metal surfaces, typically at about 1600xc2x0 F. (about 870xc2x0 C.) including boron, boron oxides or metal borides (U.S. Pat. No. 4,555,326) boron oxides, metal borides, and boric acid (U.S. Pat. No. 4,724,064) ammonium borate U.S. Pat. No. 4,680,421); and boric acid, boric oxide and borax (U.S. Pat. No. 3,661,820). These patents fail to teach the use of the chromate and dichromate compounds of the present invention and fail to teach the use of such compounds in decoking transfer line exchangers.
Chemical Abstract Vol. 83; 30687k (of French Patent 2,202,930) teaches adding molten oxides or salts of group III (now 13), IV (now 14) and VIII (now 8, 9, and 10). The abstract does not disclose the use of the metal chromates and dichromates of the present invention nor does the abstract disclose the treatment of transfer line exchangers.
U.S. Pat. No. 2,063,596, issued Dec. 8, 1936 to I. G. Farbenindustrie Aktiengesellschaft discloses exposing compounds such as molybdenum carbonyl, tetra ethyl lead and chromyl chloride to temperatures above which they decompose to help reduce coke formation on metal surfaces. The patent does not teach the chemicals required in the present invention.
U.S. Pat. No. 5,648,178 issued Jul. 15, 1997 to Chevron Chemical Company teaches treating or coating (painting) the internal surface of a reactor system with a group VI B (now group 6) metal layer. A particularly useful metal is chromium and the chloride forms appear to be particularly useful in the paint. The patent fails to teach the group 1 or 2 metal chromates and dichromates of the present invention.
There are several papers by VNIIOS in 1994 and 1999 relating to inhibitors for coke build up in a furnace using group 1 and 2 metal acetates, carbonates, nitrates and sulphates and compounds of sulphur, phosphorous, boron, aluminum, silicon, tin antimony, lead, cadmium, siloxane, derivatives of monocarboxylic and alkylsulphonic acids. The inhibitor is continuously injected into the hydrocarbon process stream prior to the cracking section. VNIIOS also has a paper (Chem. Tech. Eur. September 1994, pp14-16) which discloses an accelerated decoking method for hydrocarbon furnace coils. The methods were developed for vinyl chloride (VCM) plants, but it was claimed to be applicable to furnace tubes of other plants where coke buildup is a problem. This process differs from conventional chemical cleaning methods because it uses an endothermic reaction and is carried out in the absence of air. Therefore, coke removal is achieved through catalytic gasification reactions, instead of combustion.
A Russian patent R.U. 2168533, issued Oct. 6, 2001 to V. A. Bushuev, reveals a periodical non-stop decoking process for tubular pyrolysis furnace coils. The process consists of two periods without switching the furnace train to decoke mode. In the first periodxe2x80x94hydrocarbon cracking, the first additive is introduced into hydrocarbon feed which optionally may contain sulfur. Typical additive components are phosphorous-containing or sulfur-containing compounds, such as KH2PO4, H3PO4, or DMS. During the second periodxe2x80x94coil decoking, another additive containing either alkali or alkali-earth metal compounds, such as MgCl2, MgSO4, Mg(OCOCH3)2 is introduced into hydrocarbon feed to promote online coke gasification from the coil surfaces. Again, this invention fails to reveal the use of group 1 or 2 metal chromates or dichromates to enhance decoke operation in TLE.
The present invention provides a process of treating transfer line exchangers in an ethylene cracker comprising injecting up to 15 wt % based on the stream entering the transfer line exchanger of a solution consisting of a polar solvent and up to 80 weight % of a solute composition comprising
(i) from 10 wppm to 100 wt % of one or more group 1 or 2 metal chromates and dichromates;
(ii) from 0 wppm to 40 wt % of one or more group 1, 2 and 7 metal carbonates;
(iii) from 0 to 30 wt % of one or more group 1 or 2 manganates or permanganates;
(iv) from 0 to 20 wt % of one or more group 1 and 2 metal acetates and oxalates;
(v) from 0 to 1 wt % of one or more group 6 or 7 acetates or oxalates; and
(vi) from 0 to 1 wt % of one or more group 1 and 2 metal hydroxides, into a carrier stream comprising an inert gas, or air, or process steam or mixtures thereof injected at one or more points between the outlet of the radiant coils and the inlet of said transfer line exchanger at a temperature from 300xc2x0 C. to 750xc2x0 C. during a decoking operation of said ethylene cracker for a period of time not less than 1 second.